I’m far too laidback to maintain a cedar hedge*, that is a hedge of Thuja occidentalis, also called arborvitae. I leave that for much harder-working gardeners than myself. But if you’re stuck with a cedar hedge, here’s some information on how to prune it.
* I prefer an informal hedge made of deciduous shrubs selected so that their maximum height and width meet my needs. That way, I never need to prune!
The ideal time to trim a cedar hedge is from early to late June, depending on your climate, after the lilacs have finished blooming. At this point, it has pretty much finished growing for the year, but its new growth will still be a lighter green than the previous year’s stems. And that’s important, because ideally you would only prune into the current year’s growth. You can also prune later in the season, as late as early September, but at that point distinguishing between new growth and older growth is no longer as easy, as it will have lost is “spring green” color.
When you prune, your goal will be to remove one third to two thirds of the length of the current year’s shoots. You can use either a manual or motorized hedge trimmer to do the job. Young hedges will need less pruning (you want them to grow in height, after all), but once the desired height is achieved, you’ll need to prune annually.
To trace a perfect line (warning: this is far too much precision for a laidback gardener!), insert a tall stake at each end of the hedgerow, stretch a cord between each, pull it taut, then adjust the cord to just the right height using a level. Now simply follow the cord with trimmer… without cutting the cord, of course. Best of luck! (No way you’ll catch me doing that!)
Giving Your Hedge an Appropriate Shape
For a hedge to remain green from the bottom up, it has to be broader at base than at the top. That way, its bottom leaves won’t be shaded by the growth above and will therefore receive their share of sunlight rather than thinning out over time and losing their foliage. Also the top should be rounded or pyramidal so it will shed snow and ice more readily and therefore not split open on those tough winter days.
Can You Reduce A Cedar Hedge?
Ideally, you’d control the height of your cedar hedge by trimming it annually, never cutting beyond the current year’s growth. However, if your hedge has been just a bit neglected – say for no more than two years – it is possible to slightly reduce its height and width by pruning more severely while still remaining in the green part. Never cut into the wood over two years old (brown wood), as cedars will not regrow well from old wood.
But If It’s Been Severely Neglected?
If your cedar hedge has become far too wide or far too tall (and that happens quickly if you fail to trim it annually, remembering that cedars will grow from 1 to 3 feet/30-90 cm a year!), pruning it back to a more acceptable size will probably not be possible. Unlike a non-conifer hedge that you can cut back almost to the ground if necessary and that will then grow back fairly, if you prune into the old wood of a cedar hedge, any regrowth will be irregular, leaving it with a open top and sides full of dead patches that will take years to recuperate, if indeed they ever do. You’ll find it is better tear the hedge out and start all over.
However, while you can’t realistically make a fat hedge much thinner, you can shorten a tall hedge considerably, as long as you don’t expect perfect results in the near future. To try this, prune the hedge at least a foot (30 cm) lower than you really want it to be. At first this will leave a hedge with green sides and a dead top. However if you prune carefully every year, allowing the hedge to grow about 2 inches (5 cm) taller each season, you will gradually be able to encourage the side branches to grow inward and start to fill in the dead center. The gaping hole at the top will then eventually fill in, leaving you with an acceptable hedge. Remember though that this can take years.
The moral to this story is therefore: if you plant a cedar hedge, you have to trim it every year. No buts about it!